It is known to equip a mechanical device, such as a motor including a rotor and a stator, with a sensor-bearing unit for tracking the rotation of the rotor relative to the stator. Other mechanical devices including a mobile part and a stationary part may be equipped with such a sensor device, for example power steering, alternator and rack rails devices. Depending on the mechanical device, the mobile part may move in rotation or in translation. Thus, the sensor device may include a rotation angle sensor or a linear sensor.
In other words, each mechanical device is equipped with a dedicated sensor device, implying a specific indexation to provide effective measurements.
Some customers using sensor-bearing units do not have a lot of experience and knowledge on the motor control strategies and therefore use off-the-shelve solutions. These solutions often only accept a very precise indexation of the sensor-bearing output signals with the rotor magnetic field. Typically, an error of only ±8 degrees may be accepted between the zero-index of the output signals and the zero-index of the rotor if this error may be adjusted by a controller, and an error of only ±0.5 degrees may be accepted in case there is no adjustment in a controller.
Mechanically, the zero-index can be set in two ways: pre-indexing the rotor with the shaft (achieved by the motor manufacturer) and/or pre-indexing the shaft with the sensor (achieved by motor manufacturer or sensor manufacturer). These methods are difficult and costly to implement since they involve a precise mechanical indexation.
For instance, it is known to mount a sensor-bearing unit with a degree of freedom on a shaft, allowing the sensor to rotate in order to mechanically shift the output signals with respect to the rotor. A rotation of the sensor up to 180 degrees around the rotor may be necessary to provide correct shift. Such a method is difficult to implement and requires specific tooling for precise indexation of mechanical angular offset.
WO-A-2007/077389, corresponding to the preamble of claim 1, describes a system for detecting the angular position of a rotating element with respect to a non-rotating element. Sensors are fixed to the rotating or the non-rotating element and their output signals are processed to generate a differential signal.
WO-A-2010/082086 describes a method and a device for sensing an absolute rotation angle of an electric energy assisted steering, comprising a rotor mobile around a rotation axis and through several revolutions. The sensing device is connected to an electronic control unit (ECU) having a signal processor, a converter, a calculating processor and an integrating unit. ECU adjusts the offset and the gain of output signal of the sensing device.
WO-A-2011/070391 describes a method and a device for estimating the angular position of a rotating device with respect to a stationary device. The measuring device includes several angle sensors connected to an electronic control unit (ECU) for generating and treating signals to estimate the angular position of the rotating device.